26.5 Unordered associative containers [unord]
mapped_type& operator[](const key_type& k);
Effects: Equivalent to: return try_emplace(k).first->second;
mapped_type& operator[](key_type&& k);
Effects: Equivalent to: return try_emplace(move(k)).first->second;
mapped_type& at(const key_type& k);
const mapped_type& at(const key_type& k) const;
Returns: A reference to
x.second, where
x is the (unique) element whose key is equivalent to
k. Throws: An exception object of type
out_of_range if no such element is present
.